Transparent conductive films are important components that, among other applications, are used as electrodes for various electronic devices such as liquid crystal displays (LCDs), plasma displays (PDs), touch panels, organic light-emitting diodes (OLEDs) and photovoltaics. Transparent conductive films also find use in electrostatic discharge (ESD) applications, though the required range of surface resistivity is much higher, 109 to 1012 Ω/sq, compared to that required for the display industry, usually lower than 103 Ω/sq. Indium tin oxide (ITO) is currently the choice for such films as it exhibits very unique properties combining optical transparency (approximately 90 percent) and low electrical resistivity (100 Ω/sq). However, indium tin oxide (ITO) poses several problems when improved flexibility of the final device is desired/needed. Thin ITO layers on flexible substrates such as PET are brittle and easily shattered when bent and rolled repeatedly. This typically results in lost conductivity at very small strains, for example at 2.5 percent (see, e.g., Danran R. Cairns et al.; Applied Physics Letters; 2000; Vol. 76; No. 11; pp. 1425 to 1427). This problem becomes more severe when stretchability to large strains coupled with bendability and rollability is required. There are several situations where stretchability is desired alongside with flexibility in electronic materials including displays. In a continuous roll-to-roll (R2R) process of display manufacturing, it is possible the substrate will be exposed to high levels of strain, especially during thermoforming In addition, future displays, solar cells, wearable electronics and skin attached implant sensors require stretchability and more desirably subsequent recovery after cessation of stress.
Several studies describe the preparation of transparent electrically conductive flexible films where ITO is completely eliminated. Carbon nanotubes, graphenes and intrinsically conducting and solution processable polymers such as PEDOT-PSS have been the most commonly utilized materials. (see, e.g.: (1) Zhuangchun Wu et al.; Science; 2004; Vol. 305; pp. 1273 to 1276; (2) Keun Soo Kim et al.; Nature, 2009; Vol. 457; pp. 706 to 710; (3) Yue Sun et al.; Synthetic Metals; 1996; Vol. 82; pp. 35 to 40; (4) Jinyeol Kim et al.; Synthetic Metals; 2003; Vol. 139; pp. 485 to 489; (5) Jyongsik Jang et al.; Advanced Functional Materials; 2005; Vol. 15; No. 3; pp. 494 to 502; (6) Jianyong Ouyang et al.; Advanced Materials; 2006; Vol. 18; pp. 2141 to 2144.)
Both carbon based films, for example, carbon nanotube and graphenes, rely on the formation of a continuous thin carbon film layer on top of a flexible substrate. For the preparation of these films, a transferring step, a complication for a continuous roll-to-roll (R2R) process, is typically required. Sputtering, reactive evaporation, chemical vapor deposition, the sol-gel processes and spray coating are other processes used to coat a thin continuous layer of electrically conducting layer on flexible substrates. Although such organic thin layers of electrically conductive carbon based materials deposited on flexible polymer substrates can tolerate repeated bending and flexing as demonstrated recently, the tolerance of such thin coating layers to high levels of stretching while maintaining conductivity is more challenging.
Thus, there is a need in the art for improved stretchable, flexible, transparent conductive polymer films, and a method for making same.